Mechanical memory stored through epigenetic remodeling reduces cell therapeutic potential

Dynamic substrate topographies drive actin- and vimentin-mediated nuclear mechanoprotection events in human fibroblasts

BACKGROUND

Dynamic physical changes in the extracellular environment of living tissues present a mechanical challenge for resident cells that can lead to damage to the nucleus, genome, and DNA. Recent studies have started to uncover nuclear mechanoprotection mechanisms that prevent excessive mechanical deformations of the nucleus. Here, we hypothesized that dynamic topographical changes in the cellular environment can be mechanically transmitted to the nucleus and trigger nuclear mechanoprotection events. We tested this using a photoresponsive hydrogel whose surface topography can be reversibly changed on demand upon light illumination, allowing us to subject cells to recurring microscale topographical changes.

RESULTS

With each recurring topographical change, fibroblasts were found to increasingly compact and relocate their nuclei away from the dynamic regions of the hydrogel. These cell-scale reorganization events were accompanied by an increase of global histone acetylation and decreased methylation in cells on the dynamic topographies, resulting in a minimization of DNA strand breakage. We further found that these nuclear mechanoprotection events were mediated by both vimentin intermediate filaments and the actin cytoskeleton.

CONCLUSIONS

Together, these data reveal that fibroblasts actively protect their nuclei in the presence of dynamic topographical changes through cytoskeleton-mediated mechanisms. Broadly, these results stress the importance of gaining a deeper fundamental understanding of the cellular mechanoresponse under dynamically changing conditions.

Recent Research on Chondrocyte Dedifferentiation and Insights for Regenerative Medicine

Chondrocytes maintain the balance of the extracellular matrix by synthesizing glycoproteins, collagen, proteoglycans and hyaluronic acid. Chondrocyte dedifferentiation refers to a process in which chondrocytes lose their mature differentiated phenotype and transform into a fibroblast-like morphology with fewer differentiated stages and inferior function under external stimulation. The important mechanism of homeostasis loss in osteoarthritis (OA) is a change in the chondrocyte phenotype. The dedifferentiation markers of chondrocytes are upregulated in OA, and the pathogenic factors related to OA have also been shown to enhance chondrocyte dedifferentiation. In this review, we compile recent studies on chondrocyte dedifferentiation, with an emphasis on potential markers and the underlying mechanisms of dedifferentiation, as well as the current research progress in inhibiting dedifferentiation or achieving redifferentiation. A deep understanding of chondrocyte dedifferentiation would not only support the pathogenesis of OA theoretically but also provide insightful ideas for regenerative medicine to manipulate the functional phenotype of cells.

Cellular mechanical memory: a potential tool for mesenchymal stem cell-based therapy

Recent studies have shown that mechanical properties such as extracellular matrix stiffness, fluid flow, weight loading, compression, and stretching can affect cellular functions. Some examples of cell responses to mechanical properties could be the migration of cancer cells from rigid to soft surfaces or the differentiation of fibroblasts into myofibroblasts. Cellular responses to mechanical changes can modify the insertion of proteins in the extracellular matrix (ECM), causing an increase in tissue stiffness with functional consequences. In general, mechanical and physical factors can affect any kind of cell phenotype in culture conditions and in vivo tissues. Cells sense mechanical stimuli by applying force and restructuring their shape and functions in response to the resistance of the stimuli. Furthermore, mechanical triggers can develop a "memory" for altering cellular plasticity and adaptation. This phenomenon is called cellular mechanical memory (CMM), a singular feature of mesenchymal stem cells (MSCs). Controlled targeting of CMM may resolve the scarcity of viable cells needed for cell based therapy (CBT) and implement studies concerning cancer research, fibrosis, and senescence. This review focusses on cells from the mesodermal lineage, such as MSCs, fibroblasts and chondrocytes, and the role of CMM as a potential target for CBT.

Mechanical Forces, Nucleus, Chromosomes, and Chromatin

Individual cells and cells within the tissues and organs constantly face mechanical challenges, such as tension, compression, strain, shear stress, and the rigidity of cellular and extracellular surroundings. Besides the external mechanical forces, cells and their components are also subjected to intracellular mechanical forces, such as pulling, pushing, and stretching, created by the sophisticated force-generation machinery of the cytoskeleton and molecular motors. All these mechanical stressors switch on the mechanotransduction pathways, allowing cells and their components to respond and adapt. Mechanical force-induced changes at the cell membrane and cytoskeleton are also transmitted to the nucleus and its nucleoskeleton, affecting nucleocytoplasmic transport, chromatin conformation, transcriptional activity, replication, and genome, which, in turn, orchestrate cellular mechanical behavior. The memory of mechanoresponses is stored as epigenetic and chromatin structure modifications. The mechanical state of the cell in response to the acellular and cellular environment also determines cell identity, fate, and immune response to invading pathogens. Here, we give a short overview of the latest developments in understanding these processes, emphasizing their effects on cell nuclei, chromosomes, and chromatin.

Feeling the force from within - new tools and insights into nuclear mechanotransduction

The ability of cells to sense and respond to mechanical signals is essential for many biological processes that form the basis of cell identity, tissue development and maintenance. This process, known as mechanotransduction, involves crucial feedback between mechanical force and biochemical signals, including epigenomic modifications that establish transcriptional programs. These programs, in turn, reinforce the mechanical properties of the cell and its ability to withstand mechanical perturbation. The nucleus has long been hypothesized to play a key role in mechanotransduction due to its direct exposure to forces transmitted through the cytoskeleton, its role in receiving cytoplasmic signals and its central function in gene regulation. However, parsing out the specific contributions of the nucleus from those of the cell surface and cytoplasm in mechanotransduction remains a substantial challenge. In this Review, we examine the latest evidence on how the nucleus regulates mechanotransduction, both via the nuclear envelope (NE) and through epigenetic and transcriptional machinery elements within the nuclear interior. We also explore the role of nuclear mechanotransduction in establishing a mechanical memory, characterized by a mechanical, epigenetic and transcriptomic cell state that persists after mechanical stimuli cease. Finally, we discuss current challenges in the field of nuclear mechanotransduction and present technological advances that are poised to overcome them.

Mechanical signatures in cancer metastasis

The cancer metastatic cascade includes a series of mechanical barrier-crossing events, involving the physical movement of cancer cells from their primary location to a distant organ. This review describes the physical changes that influence tumour proliferation, progression, and metastasis. We identify potential mechanical signatures at every step of the metastatic cascade and discuss some latest mechanobiology-based therapeutic interventions to highlight the importance of interdisciplinary approaches in cancer diagnosis and treatment.

A human stem cell-derived model reveals pathologic extracellular matrix remodeling in diabetic podocyte injury

Diabetic nephropathy results from chronic (or uncontrolled) hyperglycemia and is the leading cause of kidney failure. The kidney's glomerular podocytes are highly susceptible to diabetic injury and subsequent non-reversible degeneration. We generated a human induced pluripotent stem (iPS) cell-derived model of diabetic podocytopathy to investigate disease pathogenesis and progression. The model recapitulated hallmarks of podocytopathy that precede proteinuria including retraction of foot processes and podocytopenia (detachment from the extracellular matrix (ECM)). Moreover, hyperglycemia-induced injury to podocytes exacerbated remodeling of the ECM. Specifically, mature podocytes aberrantly increased expression and excessively deposited collagen (IV)α1α1α2 that is normally abundant in the embryonic glomerulus. This collagen (IV) imbalance coincided with dysregulation of lineage-specific proteins, structural abnormalities of the ECM, and podocytopenia - a mechanism not shared with endothelium and is distinct from drug-induced injury. Intriguingly, repopulation of hyperglycemia-injured podocytes on decellularized ECM scaffolds isolated from healthy podocytes attenuated the loss of synaptopodin (a mechanosensitive protein associated with podocyte health). These results demonstrate that human iPS cell-derived podocytes can facilitate in vitro studies to uncover the mechanisms of chronic hyperglycemia and ECM remodeling and guide disease target identification.

Trusting the forces of our cell lines

Cells isolated from their native tissues and cultured in vitro face different selection pressures than those cultured in vivo. These pressures induce a profound transformation that reshapes the cell, alters its genome, and transforms the way it senses and generates forces. In this perspective, we focus on the evidence that cells cultured on conventional polystyrene substrates display a fundamentally different mechanobiology than their in vivo counterparts. We explore the role of adhesion reinforcement in this transformation and to what extent it is reversible. We argue that this mechanoadaptation is often understood as a mechanical memory. We propose some strategies to mitigate the effects of on-plastic culture on mechanobiology, such as organoid-inspired protocols or mechanical priming. While isolating cells from their native tissues and culturing them on artificial substrates has revolutionized biomedical research, it has also transformed cellular forces. Only by understanding and controlling them, we can improve their truthfulness and validity.

Fibroblast and myofibroblast activation in normal tissue repair and fibrosis

The term 'fibroblast' often serves as a catch-all for a diverse array of mesenchymal cells, including perivascular cells, stromal progenitor cells and bona fide fibroblasts. Although phenotypically similar, these subpopulations are functionally distinct, maintaining tissue integrity and serving as local progenitor reservoirs. In response to tissue injury, these cells undergo a dynamic fibroblast-myofibroblast transition, marked by extracellular matrix secretion and contraction of actomyosin-based stress fibres. Importantly, whereas transient activation into myofibroblasts aids in tissue repair, persistent activation triggers pathological fibrosis. In this Review, we discuss the roles of mechanical cues, such as tissue stiffness and strain, alongside cell signalling pathways and extracellular matrix ligands in modulating myofibroblast activation and survival. We also highlight the role of epigenetic modifications and myofibroblast memory in physiological and pathological processes. Finally, we discuss potential strategies for therapeutically interfering with these factors and the associated signal transduction pathways to improve the outcome of dysregulated healing.

GelMA hydrogel dual photo-crosslinking to dynamically modulate ECM stiffness

The dynamic nature of the extracellular matrix (ECM), particularly its stiffness, plays a pivotal role in cellular behavior, especially after myocardial infarction (MI), where cardiac fibroblasts (cFbs) are key in ECM remodeling. This study explores the effects of dynamic stiffness changes on cFb activation and ECM production, addressing a gap in understanding the dynamics of ECM stiffness and their impact on cellular behavior. Utilizing gelatin methacrylate (GelMA) hydrogels, we developed a model to dynamically alter the stiffness of cFb environment through a two-step photocrosslinking process. By inducing a quiescent state in cFbs with a TGF-β inhibitor, we ensured the direct observation of cFbs-responses to the engineered mechanical environment. Our findings demonstrate that the mechanical history of substrates significantly influences cFb activation and ECM-related gene expression. Cells that were initially cultured for 24 h on the soft substrate remained more quiescent when the hydrogel was stiffened compared to cells cultured directly to a stiff static substrate. This underscores the importance of past mechanical history in cellular behavior. The present study offers new insights into the role of ECM stiffness changes in regulating cellular behavior, with significant implications for understanding tissue remodeling processes, such as in post-MI scenarios.

Insights into the mechanobiology of cancer metastasis via microfluidic technologies

During cancer metastasis, cancer cells will encounter various microenvironments with diverse physical characteristics. Changes in these physical characteristics such as tension, stiffness, viscosity, compression, and fluid shear can generate biomechanical cues that affect cancer cells, dynamically influencing numerous pathophysiological mechanisms. For example, a dense extracellular matrix drives cancer cells to reorganize their cytoskeleton structures, facilitating confined migration, while this dense and restricted space also acts as a physical barrier that potentially results in nuclear rupture. Identifying these pathophysiological processes and understanding their underlying mechanobiological mechanisms can aid in the development of more effective therapeutics targeted to cancer metastasis. In this review, we outline the advances of engineering microfluidic devices in vitro and their role in replicating tumor microenvironment to mimic in vivo settings. We highlight the potential cellular mechanisms that mediate their ability to adapt to different microenvironments. Meanwhile, we also discuss some important mechanical cues that still remain challenging to replicate in current microfluidic devices in future direction. While much remains to be explored about cancer mechanobiology, we believe the developments of microfluidic devices will reveal how these physical cues impact the behaviors of cancer cells. It will be crucial in the understanding of cancer metastasis, and potentially contributing to better drug development and cancer therapy.

Epigenetic Priming Enhances Chondrogenic Potential of Expanded Chondrocytes

Expansion of chondrocytes presents a major obstacle in the cartilage regeneration procedure, such as matrix-induced autologous chondrocyte implantation. Dedifferentiation of chondrocytes during the expansion process leads to the emergence of a fibrotic (chondrofibrotic) phenotype that decreases the chondrogenic potential of the implanted cells. We aim to (1) determine the extent that chromatin architecture of H3K27me3 and H3K9me3 remodels during dedifferentiation and persists after the transfer to a three-dimensional (3D) culture; and (2) to prevent this persistent remodeling to enhance the chondrogenic potential of expanded bovine chondrocytes, used as a model system. Chromatin architecture remodeling of H3K27me3 and H3K9me3 was observed at 0 population doublings, 8 population doublings, and 16 population doublings (PD16) in a two-dimensional (2D) culture and after encapsulation of the expanded chondrocytes in a 3D hydrogel culture. Chondrocytes were treated with inhibitors of epigenetic modifiers (epigenetic priming) for PD16 and then encapsulated in 3D hydrogels. Chromatin architecture of chondrocytes and gene expression were evaluated before and after encapsulation. We observed a change in chromatin architecture of epigenetic modifications H3K27me3 and H3K9me3 during chondrocyte dedifferentiation. Although inhibiting enzymes that modify H3K27me3 and H3K9me3 did not alter the dedifferentiation process in 2D culture, applying these treatments during the 2D expansion did increase the expression of select chondrogenic genes and protein deposition of type II collagen when transferred to a 3D environment. Overall, we found that epigenetic priming of expanded bovine chondrocytes alters the cell fate when chondrocytes are later encapsulated into a 3D environment, providing a potential method to enhance the success of cartilage regeneration procedures.

A Protein-Adsorbent Hydrogel with Tunable Stiffness for Tissue Culture Demonstrates Matrix-Dependent Stiffness Responses

Although tissue culture plastic has been widely employed for cell culture, the rigidity of plastic is not physiologic. Softer hydrogels used to culture cells have not been widely adopted in part because coupling chemistries are required to covalently capture extracellular matrix (ECM) proteins and support cell adhesion. To create an in vitro system with tunable stiffnesses that readily adsorbs ECM proteins for cell culture, we present a novel hydrophobic hydrogel system via chemically converting hydroxyl residues on the dextran backbone to methacrylate groups, thereby transforming non-protein adhesive, hydrophilic dextran to highly protein adsorbent substrates. Increasing methacrylate functionality increases the hydrophobicity in the resulting hydrogels and enhances ECM protein adsorption without additional chemical reactions. These hydrophobic hydrogels permit facile and tunable modulation of substrate stiffness independent of hydrophobicity or ECM coatings. Using this approach, we show that substrate stiffness and ECM adsorption work together to affect cell morphology and proliferation, but the strengths of these effects vary in different cell types. Furthermore, we reveal that stiffness mediated differentiation of dermal fibroblasts into myofibroblasts is modulated by the substrate ECM. Our material system demonstrates remarkable simplicity and flexibility to tune ECM coatings and substrate stiffness and study their effects on cell function.

Mechanotransduction through protein stretching

Cells sense and respond to subtle changes in their physicality, and via a myriad of different mechanosensitive processes, convert these physical cues into chemical and biochemical signals. This process, called mechanotransduction, is possible due to a highly sophisticated machinery within cells. One mechanism by which this can occur is via the stretching of mechanosensitive proteins. Stretching proteins that contain force-dependent regions results in altered geometry and dimensions of the connections, as well as differential spatial organization of signals bound to the stretched protein. The purpose of this mini-review is to discuss some of the intense recent activity in this area of mechanobiology that strives to understand how protein stretching can influence signaling outputs and cellular responses.

Passaged Articular Chondrocytes From the Superficial Zone and Deep Zone Can Regain Zone-Specific Properties After Redifferentiation

BACKGROUND

Bioengineered cartilage is a developing therapeutic to repair cartilage defects. The matrix must be rich in collagen type II and aggrecan and mechanically competent, withstanding compressive and shearing loads. Biomechanical properties in native articular cartilage depend on the zonal architecture consisting of 3 zones: superficial, middle, and deep. The superficial zone chondrocytes produce lubricating proteoglycan-4, whereas the deep zone chondrocytes produce collagen type X, which allows for integration into the subchondral bone. Zonal and chondrogenic expression is lost after cell number expansion. Current cell-based therapies have limited capacity to regenerate the zonal structure of native cartilage.

HYPOTHESIS

Both passaged superficial and deep zone chondrocytes at high density can form bioengineered cartilage that is rich in collagen type II and aggrecan; however, only passaged superficial zone-derived chondrocytes will express superficial zone-specific proteoglycan-4, and only passaged deep zone-derived chondrocytes will express deep zone-specific collagen type X.

STUDY DESIGN

Controlled laboratory study.

METHODS

Superficial and deep zone chondrocytes were isolated from bovine joints, and zonal subpopulations were separately expanded in 2-dimensional culture. At passage 2, superficial and deep zone chondrocytes were seeded, separately, in scaffold-free 3-dimensional culture within agarose wells and cultured in redifferentiation media.

RESULTS

Monolayer expansion resulted in loss of expression for proteoglycan-4 and collagen type X in passaged superficial and deep zone chondrocytes, respectively. By passage 2, superficial and deep zone chondrocytes had similar expression for dedifferentiated molecules collagen type I and tenascin C. Redifferentiation of both superficial and deep zone chondrocytes led to the expression of collagen type II and aggrecan in both passaged chondrocyte populations. However, only redifferentiated deep zone chondrocytes expressed collagen type X, and only redifferentiated superficial zone chondrocytes expressed and secreted proteoglycan-4. Additionally, redifferentiated deep zone chondrocytes produced a thicker and more robust tissue compared with superficial zone chondrocytes.

CONCLUSION

The recapitulation of the primary phenotype from passaged zonal chondrocytes introduces a novel method of functional bioengineering of cartilage that resembles the zone-specific biological properties of native cartilage.

CLINICAL RELEVANCE

The recapitulation of the primary phenotype in zonal chondrocytes could be a possible method to tailor bioengineered cartilage to have zone-specific expression.

Advances in cancer mechanobiology: Metastasis, mechanics, and materials

Within the tumor microenvironment (TME), tumor cells are exposed to numerous mechanical forces, both internally and externally, which contribute to the metastatic cascade. From the initial growth of the tumor to traveling through the vasculature and to the eventual colonization of distant organs, tumor cells are continuously interacting with their surroundings through physical contact and mechanical force application. The mechanical forces found in the TME can be simplified into three main categories: (i) shear stress, (ii) tension and strain, and (iii) solid stress and compression. Each force type can independently impact tumor growth and progression. Here, we review recent bioengineering strategies, which have been employed to establish the connection between mechanical forces and tumor progression. While many cancers are explored in this review, we place great emphasis on cancers that are understudied in their response to mechanical forces, such as ovarian and colorectal cancers. We discuss the major steps of metastatic transformation and present novel, recent advances in model systems used to study how mechanical forces impact the study of the metastatic cascade. We end by summarizing systems that incorporate multiple forces to expand the complexity of our understanding of how tumor cells sense and respond to mechanical forces in their environment. Future studies would also benefit from the inclusion of time or the aspect of mechanical memory to further enhance this field. While the knowledge of mechanical forces and tumor metastasis grows, developing novel materials and in vitro systems are essential to providing new insight into predicting, treating, and preventing cancer progression and metastasis.

Mechanical forces amplify TCR mechanotransduction in T cell activation and function

Adoptive T cell immunotherapies, including engineered T cell receptor (eTCR) and chimeric antigen receptor (CAR) T cell immunotherapies, have shown efficacy in treating a subset of hematologic malignancies, exhibit promise in solid tumors, and have many other potential applications, such as in fibrosis, autoimmunity, and regenerative medicine. While immunoengineering has focused on designing biomaterials to present biochemical cues to manipulate T cells ex vivo and in vivo, mechanical cues that regulate their biology have been largely underappreciated. This review highlights the contributions of mechanical force to several receptor-ligand interactions critical to T cell function, with central focus on the TCR-peptide-loaded major histocompatibility complex (pMHC). We then emphasize the role of mechanical forces in (i) allosteric strengthening of the TCR-pMHC interaction in amplifying ligand discrimination during T cell antigen recognition prior to activation and (ii) T cell interactions with the extracellular matrix. We then describe approaches to design eTCRs, CARs, and biomaterials to exploit TCR mechanosensitivity in order to potentiate T cell manufacturing and function in adoptive T cell immunotherapy.

Implications of Cellular Mechanical Memory in Bioengineering

The ability to maintain and differentiate cells in vitro is critical to many advances in the field of bioengineering. However, on traditional, stiff (E ≈ GPa) culture substrates, cells are subjected to sustained mechanical stress that can lead to phenotypic changes. Such changes may remain even after transferring the cells to another scaffold or engrafting them in vivo and bias the outcomes of the biological investigation or clinical treatment. This persistence─or mechanical memory─was initially observed for sustained myofibroblast activation of pulmonary fibroblasts after culturing them on stiff (E ≈ 100 kPa) substrates. Aspects of mechanical memory have now been described in many in vitro contexts. In this Review, we discuss the stiffness-induced effectors of mechanical memory: structural changes in the cytoskeleton and activity of transcription factors and epigenetic modifiers. We then focus on how mechanical memory impacts cell expansion and tissue regeneration outcomes in bioengineering applications relying on prolonged 2D plastic culture, such as stem cell therapies and disease models. We propose that alternatives to traditional cell culture substrates can be used to mitigate or erase mechanical memory and improve the efficiency of downstream cell-based bioengineering applications.

Mechanically induced alterations in chromatin architecture guide the balance between cell plasticity and mechanical memory

Phenotypic plasticity, or adaptability, of a cell determines its ability to survive and function within changing cellular environments. Changes in the mechanical environment, ranging from stiffness of the extracellular matrix (ECM) to physical stress such as tension, compression, and shear, are critical environmental cues that influence phenotypic plasticity and stability. Furthermore, an exposure to a prior mechanical signal has been demonstrated to play a fundamental role in modulating phenotypic changes that persist even after the mechanical stimulus is removed, creating stable mechanical memories. In this mini review, our objective is to highlight how the mechanical environment alters both phenotypic plasticity and stable memories through changes in chromatin architecture, mainly focusing on examples in cardiac tissue. We first explore how cell phenotypic plasticity is modulated in response to changes in the mechanical environment, and then connect the changes in phenotypic plasticity to changes in chromatin architecture that reflect short-term and long-term memories. Finally, we discuss how elucidating the mechanisms behind mechanically induced chromatin architecture that lead to cell adaptations and retention of stable mechanical memories could uncover treatment methods to prevent mal-adaptive permanent disease states.